Intra-prediction apparatus for removing a directional intra-prediction mode from a set of predetermined directional intra-prediction modes

ABSTRACT

An intra-prediction method includes: determining, by an intra-prediction apparatus, a directional intra-prediction mode for a rectangular video coding block from an extended set of directional intra-prediction modes, wherein the extended set of directional intra-prediction modes includes extended directional intra-prediction modes and conventional directional intra-prediction modes and determining, by the intra-prediction apparatus, a plurality of available reference samples based on a direction of the directional intra-prediction mode. The method further includes: intra-predicting, by the intra-prediction apparatus, pixel values of pixels of the rectangular video coding block based on the plurality of available reference samples.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Application No. 17/334,267,filed on May 28, 2021, which is a continuation of U.S. Application No.16/449,121, filed on Jun. 21, 2019, now U.S. Patent No. 11,025,908,which is a continuation of International Application No.PCT/RU2016/000915, filed on Dec. 23, 2016. All of the aforementionedpatent applications are hereby incorporated by reference in theirentireties.

TECHNICAL FIELD

Generally, the present disclosure relates to the field of video coding.The present disclosure relates to an apparatus for directionalintra-prediction of a video coding block as well as an encodingapparatus and a decoding apparatus comprising such an intra-predictionapparatus.

BACKGROUND

Digital video communication and storage applications are implemented bya wide range of digital devices, e.g. digital cameras, cellular radiotelephones, laptops, broadcasting systems, video teleconferencingsystems, etc. One of the most important and challenging tasks of theseapplications is video compression. The task of video compression iscomplex and is constrained by two contradicting parameters: compressionefficiency and computational complexity. Video coding standards, such asITU-T H.264/AVC or ITU-T H.265/HEVC, provide a good tradeoff betweenthese parameters. For that reason support of video coding standards is amandatory requirement for almost any video compression application.

The state-of-the-art video coding standards are based on partitioning ofa source picture into video coding blocks (or short blocks). Processingof these blocks depends on their size, spatial position and a codingmode specified by an encoder. Coding modes can be classified into twogroups according to the type of prediction: intra- and inter-predictionmodes. Intra-prediction modes use pixels of the same picture (alsoreferred to as frame or image) to generate reference samples tocalculate the prediction values for the pixels of the block beingreconstructed. Intra-prediction is also referred to as spatialprediction. Inter-prediction modes are designed for temporal predictionand uses reference samples of previous or next pictures to predictpixels of the block of the current picture. After a prediction stage,transform coding is performed for a prediction error that is thedifference between an original signal and its prediction. Then, thetransform coefficients and side information are encoded using an entropycoder (e.g., CABAC for AVC/H.264 and HEVC/H.265). The recently adoptedITU-T H.265/HEVC standard (ISO/IEC 23008-2:2013, “Informationtechnology - High efficiency coding and media delivery in heterogeneousenvironments - Part 2: High efficiency video coding”, November 2013)declares a set of state-of-the-art video coding tools that provide areasonable tradeoff between coding efficiency and computationalcomplexity. An overview on the ITU-T H.265/HEVC standard has been givenby Gary J. Sullivan, “Overview of the High Efficiency Video Coding(HEVC) Standard”, in IEEE Transactions on Circuits and Systems for VideoTechnology, Vol. 22, No. 12, December 2012, the entire content of whichis incorporated herein by reference.

Similar to the ITU-T H.264/AVC video coding standard, the HEVC/H.265video coding standard provides for a division of the source picture intoblocks, e.g., coding units (CUs). Each of the CUs can be further splitinto either smaller CUs or prediction units (PUs). A PU can be intra- orinter-predicted according to the type of processing applied for thepixels of PU. In case of inter-prediction, a PU represents an area ofpixels that is processed by motion compensation using a motion vectorspecified for a PU. For intra prediction, the adjacent pixels ofneighbor blocks are used as reference samples to predict a currentblock. A PU specifies a prediction mode that is selected from the set ofintra-prediction modes for all the transform units (TUs) contained inthis PU. A TU can have different sizes (e.g., 4×4, 8×8, 16×16 and 32×32pixels) and can be processed in different ways. For a TU, transformcoding is performed, i.e. the prediction error is transformed with adiscrete cosine transform or a discrete sine transform (in theHEVC/H.265 standard, it is applied to intra-coded blocks) and quantized.Hence, reconstructed pixels contain quantization noise (it can becomeapparent, for examples, as blockiness between units, ringing artifactsalong with sharp edges, etc.) that in-loop filters such as DeblockingFilter (DBF), Sample Adaptive Offset (SAO) and Adaptive Loop Filter(ALF) try to suppress. The use of sophisticated prediction coding (suchas motion compensation and intra-prediction) and partitioning techniques(e.g., quadtree for CUs and PUs as well as residual quadtree for TUs inthe HEVC/H.265 standard and quadtree plus binary tree for the JEMreference software starting from version JEM-3.0) allowed thestandardization committee to significantly reduce the redundancy in PUs.

According to the HEVC/H.265 standard, the intra prediction modes asshown in FIG. 5 include a planar mode (the intra-prediction mode indexis 0), DC mode (the intra-prediction mode index is 1), and 33directional modes (the intra-prediction mode index ranges from 2 to 34,indicated by the solid lines). The set of directional intra-predictionmodes was extended up to 65 modes (almost doubled) by decreasing a stepangle between directional intra-prediction modes by a factor of 2. Thedotted lines in FIG. 5 denote the angular modes, which are introduced inthe JEM software.

For the JEM-3.0 software, a new partitioning mechanism based on bothquad-tree and binary-tree (known as QTBT) was proposed. The fundamentaldifference between the QT and QTBT partitioning mechanisms is that thelatter one enables not only square but also rectangular blocks by usingpartitioning based on both quad- and binary-tree. FIGS. 6(a) and 6(b)illustrate an example of block partitioning and a corresponding treestructure by using QTBT, wherein solid lines denote quad-treepartitioning and dashed lines denote binary-tree partitioning. In eachpartitioning node of the binary-tree, the partitioning type is indicatedby 0 (horizontal partitioning) or 1 (vertical partitioning).

Some signaling overhead and increased computational complexity at theencoder side are the price of the QTBT partitioning as compared toconventional quad-tree based partitioning used in the HEVC/H.265standard. Nevertheless, the QTBT-based partitioning is endowed withbetter segmentation properties and demonstrates significantly highercoding efficiency than the conventional quad-tree (“EE2.1: Quadtree plusbinary tree structure integration with JEM tools,” ContributionJVET-C0024 to the 3^(rd) JVET meeting, Geneva, Switzerland, May 2016 byHan Huang, Kai Zhang, Yu-Wen Huang, Shawmin Lei). However, the QTBTpartitioning has a critical problem: a set of available directionalintra-prediction modes has not been changed accordingly. Thus, theasymmetrical nature of rectangular blocks utilized by the QTBT frameworkhas not been taken into account, as shown in FIGS. 7(a) and 7(b), i.e.,the same number of reference samples are used along both shorter andlonger sides of rectangular blocks. Therefore, the number of directionalintra-prediction modes depends on neither aspect ratio of blocks noractual availability of reference samples in the current implementationof the QTBT framework.

In light of the above, there is a need for apparatuses and methods forvideo coding which allow for an efficient handling of rectangular videocoding blocks.

SUMMARY

It is an object to provide apparatuses and methods for video coding,which allow for an efficient handling of rectangular video coding blocksin conjunction with a directional intra-prediction mechanism.

The foregoing and other objects are achieved by the subject matter ofthe present disclosure.

The following disclosure employs a plurality of terms which, inembodiments, have the following meaning:

Slice - a spatially distinct region of a picture that is independentlyencoded/decoded.

Slice header - Data structure configured to signal informationassociated with a particular slice. Video coding block (or shortblock) - an M×N (M-column by N-row) array of pixels or samples (eachpixel/sample being associated with at least one pixel/sample value), oran M×N array of transform coefficients.

Coding Tree Unit (CTU) grid - a grid structure employed to partitionblocks of pixels into macroblocks for video encoding.

Coding Unit (CU) - a coding block of luma samples, two correspondingcoding blocks of chroma samples of an image that has three samplearrays, or a coding block of samples of a monochrome picture or apicture that is coded using three separate color planes and syntax usedto code the samples.

Picture Parameter Set (PPS) - a syntax structure containing syntaxelements that apply to zero or more entire coded pictures as determinedby a syntax element found in each slice segment header. SequenceParameter Set (SPS) - a syntax structure containing syntax elements thatapply to zero or more entire coded video sequences as determined by thecontent of a syntax element found in the PPS referred to by a syntaxelement found in each slice segment header.

Video Parameter Set (VPS) - a syntax structure containing syntaxelements that apply to zero or more entire coded video sequences.

Prediction Unit (PU) - a prediction block of luma samples, twocorresponding prediction blocks of chroma samples of a picture that hasthree sample arrays, or a prediction block of samples of a monochromepicture or a picture that is coded using three separate color planes andsyntax used to predict the prediction block samples.

Transform Unit (TU) - a transform block of luma samples, twocorresponding transform blocks of chroma samples of a picture that hasthree sample arrays, or a transform block of samples of a monochromepicture or a picture that is coded using three separate color planes andsyntax used to predict the transform block samples.

Supplemental enhancement information (SEI) - extra information that maybe inserted into a video bit-stream to enhance the use of the video.

Luma - information indicating the brightness of an image sample.

Chroma - information indicating the color of an image sample, which maybe described in terms of red difference chroma component (Cr) and bluedifference chroma component (Cb).

Generally, the present disclosure relates to an apparatus and a methodfor improving the directional intra-prediction mechanism within the QTBTframework. The present disclosure extends a set of available directionalintra-prediction modes subject to the aspect ratio of a block to bepredicted, enables or disables some directional intra-prediction modessubject to the availability of reference samples, and signalsdirectional intra-prediction modes contained in the extended subset viamode mapping and a one-bit flag.

Embodiments described in the present disclosure provide, amongst others,the following advantages: additional coding gain after integrating thistechnique into a codec, extensive applications in hybrid video codingparadigms compatible with the HM software and the VPX video codec familyas well as in the state-of-the-art and next-generation video codingframeworks (the JEM software and VPX/AV1 video codec familyrespectively), low hardware and computational complexities at bothencoder and decoder sides, and easy implementation in such codecs thatuse conventional directional intra-prediction mechanisms.

According to a first aspect, the disclosure relates to anintra-prediction apparatus for removing a directional intra-predictionmode of a rectangular video coding block from a set of predetermineddirectional intra-prediction modes, each predetermined directionalintra-prediction mode of the set of predetermined directionalintra-prediction modes being associated with a predetermined direction,the rectangular video coding block having a first side and a secondside, a length (L_(longer)) of the first side being greater than alength (L_(shorter)) of the second side. The intra-prediction apparatuscomprises a reference sample determining unit configured to select thedirectional intra-prediction mode from the set of predetermineddirectional intra-prediction modes, to determine a plurality ofavailable reference samples on the basis of a predetermined direction ofthe directional intra-prediction mode, the available reference samplesextending along the first side of the rectangular video coding block,and to determine a length (L_(RSlonger)) associated with the pluralityof available reference samples, a directional intra-prediction moderemoving unit configured to compare the length (L_(longer)) of the firstside with the length (L_(RSlonger)) associated with the plurality ofavailable reference samples, and to remove the directionalintra-prediction mode from the set of predetermined directionalintra-prediction modes if the length (L_(RSlonger)) associated with theplurality of available reference samples is smaller than a multiple ofthe length (L_(longer)) of the first side for obtaining a reduced set ofpredetermined directional intra-prediction modes, and anintra-prediction unit configured to intra-predict pixel values of pixelsof the rectangular video coding block on the basis of the reduced set ofpredetermined directional intra-prediction modes.

In this regard, the term “direction” refers to an orientation within thevideo coding block to be used for directional intra-prediction withinthe video coding block. The term “directional range” refers to rangecovering a plurality of said directions.

In a first implementation form of the intra-prediction apparatusaccording to the first aspect as such, the directional intra-predictionmode removing unit is configured to remove the directionalintra-prediction mode from the set of predetermined directionalintra-prediction modes if the length (L_(RSlonger)) associated with theplurality of available reference samples is smaller than twice thelength (L_(longer)) of the first side. In this case, the multiple istwo.

In a second implementation form of the intra-prediction apparatusaccording to the first aspect as such or any preceding implementationform of the first aspect, the intra-prediction apparatus furthercomprises an area determining unit configured to determine anon-prediction area (S_(uncov)) within the rectangular video codingblock upon the basis of the length (L_(shorter)) of the second side andthe predetermined direction of the directional intra-prediction mode.

In a third implementation form of the intra-prediction apparatusaccording to the second implementation form of the first aspect, thearea determining unit is configured to determine the non-prediction area(S_(uncov)) within the rectangular video coding block upon the basis ofthe following equation:

$S_{\text{uncov}} = \frac{L_{\text{shorter}}^{2} \cdot \tan\gamma}{2}$

wherein S_(uncov) denotes the non-prediction area, L_(shorter) denotesthe length of the second side, and γ denotes an angle associated withthe predetermined direction of the directional intra-prediction mode.

In a fourth implementation form of the intra-prediction apparatusaccording to the second implementation form or the third implementationform of the first aspect, the area determining unit is furtherconfigured to determine a fractional non-prediction area (P_(area))within the rectangular video coding block upon the basis of the length(L_(longer)) of the first side, the length (L_(shorter)) of the secondside, and the predetermined direction of the directionalintra-prediction mode.

In a fifth implementation form of the intra-prediction apparatusaccording to the fourth implementation form of the first aspect, thearea determining unit is configured to determine the fractionalnon-prediction area (P_(area)) within the rectangular video coding blockupon the basis of the following equation:

$P_{\text{area}} = \frac{L_{\text{shorter}}}{L_{\text{longer}}} \cdot \frac{\tan\gamma}{2}$

wherein P_(area) denotes the fractional non-prediction area, L_(longer)denotes the length of the first side, L_(shorter) denotes the length ofthe second side, and γ denotes the angle associated with thepredetermined direction of the directional intra-prediction mode.

In a sixth implementation form of the intra-prediction apparatusaccording to the first aspect as such or any preceding implementationform of the first aspect, the rectangular video coding block is a codingunit (CU), a prediction unit (PU), or a transform unit (TU).

According to a second aspect, the disclosure relates to an encodingapparatus for encoding a rectangular video coding block. The encodingapparatus comprises an intra-prediction apparatus according to the firstaspect as such or any implementation form of the first aspect forproviding a predicted rectangular video coding block, and an encodingunit configured to encode the rectangular video coding block on thebasis of the predicted rectangular video coding block.

According to a third aspect, the disclosure relates to a decodingapparatus for decoding an encoded rectangular video coding block. Thedecoding apparatus comprises an intra-prediction apparatus according tothe first aspect as such or any implementation form of the first aspectfor providing a predicted rectangular video coding block, and arestoration unit configured to restore a rectangular video coding blockon the basis of an encoded rectangular video coding block and thepredicted rectangular video coding block.

According to a fourth aspect, the disclosure relates to anintra-prediction method for removing a directional intra-prediction modeof a rectangular video coding block from a set of predetermineddirectional intra-prediction modes, each predetermined directionalintra-prediction mode of the set of predetermined directionalintra-prediction modes being associated with a predetermined direction,the rectangular video coding block having a first side and a secondside, a length (L_(longer)) of the first side being greater than alength (L_(shorter)) of the second side. The intra-prediction methodcomprises selecting the directional intra-prediction mode from the setof predetermined directional intra-prediction modes, determining aplurality of available reference samples on the basis of a predetermineddirection of the directional intra-prediction mode, the availablereference samples extending along the first side of the rectangularvideo coding block, determining a length (L_(RSlonger)) associated withthe plurality of available reference samples, comparing the length(L_(longer)) of the first side with the length (L_(RSlonger)) associatedwith the plurality of available reference samples, removing thedirectional intra-prediction mode from the set of predetermineddirectional intra-prediction modes if the length (L_(RSlonger))associated with the plurality of available reference samples is smallerthan a multiple of the length (L_(longer)) of the first side forobtaining a reduced set of predetermined directional intra-predictionmodes, and intra-predicting pixel values of pixels of the rectangularvideo coding block on the basis of the reduced set of predetermineddirectional intra-prediction modes.

The intra-prediction method can be performed by the intra-predictionapparatus. Further features of the intra-prediction method directlyresult from the features or the functionality of the intra-predictionapparatus.

According to a fifth aspect, the disclosure relates to a computerprogram comprising program code for performing the method according tothe fourth aspect as such or any implementation form of the fourthaspect when executed on a computer.

The embodiments can be implemented in hardware and/or software.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect tothe following figures, wherein:

FIG. 1 shows a schematic diagram of an intra-prediction apparatus forremoving a directional intra-prediction mode of a rectangular videocoding block from a set of predetermined directional intra-predictionmodes;

FIG. 2 shows a schematic diagram of an encoding apparatus for encoding arectangular video coding block;

FIG. 3 shows a schematic diagram of a decoding apparatus for decoding anencoded rectangular video coding block;

FIG. 4 shows a schematic diagram of an intra-prediction method forremoving a directional intra-prediction mode of a rectangular videocoding block from a set of predetermined directional intra-predictionmodes;

FIG. 5 shows a schematic diagram of a video coding block illustratingdifferent directional intra-prediction modes;

FIGS. 6(a) and 6(b) illustrate an example of block partitioning and acorresponding tree structure by using quad-tree plus binary-tree (QTBT);

FIGS. 7(a) and 7(b) illustrate implementations of a directionalintra-prediction mechanism in quad-tree (QT) and quad-tree plusbinary-tree (QTBT) frameworks, respectively;

FIGS. 8(a) and 8(b) illustrate an extension of a set of directionalintra-prediction modes subject to an aspect ratio of a given rectangularvideo coding block;

FIG. 9 shows a schematic diagram illustrating an extension of a set ofdirectional intra-prediction modes subject to an aspect ratio of a givenrectangular video coding block;

FIG. 10 shows a schematic diagram illustrating a preservation of acardinality of directional intra-prediction modes subject to an aspectratio of a given rectangular video coding block;

FIG. 11 illustrates an example of block partitioning and a correspondingtree structure by using quad-tree plus binary-tree (QTBT), wherein thenumber of available reference samples along a longer side is less thanits double length in a rectangular video coding block;

FIG. 12 illustrates enabling or disabling a set of directionalintra-prediction modes subject to an availability of reference samplesof a given rectangular video coding block;

FIG. 13 illustrates a first step of a signaling mechanism for extensionof directional intra-prediction modes;

FIG. 14 illustrates a second step of a signaling mechanism for extensionof directional intra-prediction modes;

FIG. 15 illustrates a decoding process for a directional intra modeindex by applying a signaling mechanism;

FIG. 16 shows a schematic diagram illustrating an implementation of asignaling mechanism applied in an encoding apparatus;

FIG. 17 shows a schematic diagram illustrating an implementation of asignaling mechanism applied in a decoding apparatus;

FIGS. 18(a) and 18(b) show schematic diagrams illustratingimplementations of a signaling mechanism applied to the EnhancedIntra-Prediction (EIP) mechanism; and

FIG. 19 shows a schematic diagram of an encoding apparatus for encodinga rectangular video coding block comprising an intra-predictionapparatus.

In the various figures, identical reference signs will be used foridentical or at least functionally equivalent features.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, reference is made to the accompanyingdrawings, which form part of the disclosure, and in which are shown, byway of illustration, exemplary aspects of embodiments of the presentinvention. It is understood that other aspects may be utilized andstructural or logical changes may be made without departing from thescope of the present invention. The following detailed description,therefore, is not to be taken in a limiting sense, as the scope of thepresent invention is defined be the appended claims.

For instance, it is understood that a disclosure in connection with adescribed method may also hold true for a corresponding device or systemconfigured to perform the method and vice versa. For example, if amethod step is described, a corresponding device may include a unit toperform the described method step, even if such unit is not explicitlydescribed or illustrated in the figures. Further, it is understood thatthe features of the various exemplary aspects described herein may becombined with each other, unless specifically noted otherwise.

FIG. 1 shows a schematic diagram of an intra-prediction apparatus 100for removing a directional intra-prediction mode of a rectangular videocoding block from a set of predetermined directional intra-predictionmodes. Each predetermined directional intra-prediction mode of the setof predetermined directional intra-prediction modes is associated with apredetermined direction, the rectangular video coding block having afirst side and a second side, a length (L_(longer)) of the first sidebeing greater than a length (L_(shorter)) of the second side. Theintra-prediction apparatus 100 comprises a reference sample determiningunit 101 configured to select the directional intra-prediction mode fromthe set of predetermined directional intra-prediction modes, todetermine a plurality of available reference samples on the basis of apredetermined direction of the directional intra-prediction mode, theavailable reference samples extending along the first side of therectangular video coding block, and to determine a length (L_(RSlonger))associated with the plurality of available reference samples, adirectional intra-prediction mode removing unit 103 configured tocompare the length (L_(longer)) of the first side with the length(L_(RSlonger)) associated with the plurality of available referencesamples, and to remove the directional intra-prediction mode from theset of predetermined directional intra-prediction modes if the length(L_(RSlonger)) associated with the plurality of available referencesamples is smaller than a multiple of the length (L_(longer)) of thefirst side for obtaining a reduced set of predetermined directionalintra-prediction modes, and an intra-prediction unit 105 configured tointra-predict pixel values of pixels of the rectangular video codingblock on the basis of the reduced set of predetermined directionalintra-prediction modes. The intra-prediction apparatus 100 furthercomprises an area determining unit 107 configured to determine anon-prediction area (S_(uncov)) within the rectangular video codingblock upon the basis of the length (L_(shorter)) of the second side andthe predetermined direction of the directional intra-prediction mode.

FIG. 2 shows a schematic diagram of an encoding apparatus 200 forencoding a rectangular video coding block. The encoding apparatus 200comprises an intra-prediction apparatus 100 for providing a predictedrectangular video coding block, and an encoding unit 201 configured toencode the rectangular video coding block on the basis of the predictedrectangular video coding block.

FIG. 3 shows a schematic diagram of a decoding apparatus 300 fordecoding an encoded rectangular video coding block. The decodingapparatus 300 comprises an intra-prediction apparatus 100 for providinga predicted rectangular video coding block, and a restoration unit 301configured to restore a rectangular video coding block on the basis ofan encoded rectangular video coding block and the predicted rectangularvideo coding block.

FIG. 4 shows a schematic diagram of an intra-prediction method 400 forremoving a directional intra-prediction mode of a rectangular videocoding block from a set of predetermined directional intra-predictionmodes. Each predetermined directional intra-prediction mode of the setof predetermined directional intra-prediction modes is associated with apredetermined direction, the rectangular video coding block having afirst side and a second side, a length (L_(longer)) of the first sidebeing greater than a length (L_(shorter)) of the second side. Theintra-prediction method 400 comprises selecting 401 the directionalintra-prediction mode from the set of predetermined directionalintra-prediction modes, determining 403 a plurality of availablereference samples on the basis of a predetermined direction of thedirectional intra-prediction mode, the available reference samplesextending along the first side of the rectangular video coding block,determining 405 a length (L_(RSlonger)) associated with the plurality ofavailable reference samples, comparing 407 the length (L_(longer)) ofthe first side with the length (L_(RSlonger)) associated with theplurality of available reference samples, removing 409 the directionalintra-prediction mode from the set of predetermined directionalintra-prediction modes if the length (L_(RSlonger)) associated with theplurality of available reference samples is smaller than a multiple ofthe length (L_(longer)) of the first side for obtaining a reduced set ofpredetermined directional intra-prediction modes, and intra-predicting411 pixel values of pixels of the rectangular video coding block on thebasis of the reduced set of predetermined directional intra-predictionmodes.

FIG. 5 shows a schematic diagram of a video coding block illustratingdifferent directional intra-prediction modes. The intra prediction modesas shown in FIG. 5 include a planar mode (the intra-prediction modeindex is 0), DC mode (the intra-prediction mode index is 1), and 33directional modes (the intra-prediction mode index ranges from 2 to 34,indicated by the solid lines). The set of directional intra-predictionmodes was extended up to 65 modes (almost doubled) by decreasing a stepangle between directional intra-prediction modes by a factor of 2. Thedotted lines in FIG. 5 denote the angular modes, which are introduced inthe JEM software.

FIGS. 6(a) and 6(b) illustrate an example of block partitioning and acorresponding tree structure by using quad-tree plus binary-tree (QTBT),wherein solid lines denote quad-tree partitioning and dashed linesdenote binary-tree partitioning. In each partitioning node of thebinary-tree, the partitioning type is indicated by 0 (horizontalpartitioning) or 1 (vertical partitioning).

FIGS. 7(a) and 7(b) illustrate implementations of a directionalintra-prediction mechanism in quad-tree (QT) and quad-tree plusbinary-tree (QTBT) frameworks, respectively. Here, the same number ofreference samples are used along both shorter and longer sides ofrectangular blocks. Therefore, the number of directionalintra-prediction modes depends on neither aspect ratio of blocks noractual availability of reference samples in the current implementationof the QTBT framework.

FIGS. 8(a) and 8(b) illustrate an extension of a set of directionalintra-prediction modes subject to an aspect ratio of a given rectangularvideo coding block. As shown in FIG. 8(a), an aspect ratio of a squarevideo coding block is 1:1 and a set of conventional directionalintra-prediction modes is used for predicting values of a video codingblock being reconstructed. On the other hand, a rectangular video codingblock comprises shorter and longer sides, and such asymmetry can be usedto improve the current directional intra-prediction mechanism byincreasing its prediction accuracy. As illustrated in FIG. 8(b), thenumber of available directional intra-prediction modes can be increasedalong a long side.

FIG. 9 shows a schematic diagram illustrating an extension of a set ofdirectional intra-prediction modes subject to an aspect ratio of a givenrectangular video coding block. The corresponding processing steps maybe implemented by the intra-prediction apparatus 100 and/or theintra-prediction method 400. In FIG. 9 , square pixels representreference samples for intra-prediction, wherein the order ofprobabilities that the reference samples are available is: referencepixel with dots > reference pixel with stripes > reference pixel withdiagonal stripes.

The number of the newly introduced directional intra-prediction modesmay depend on the aspect ratio of the rectangular video coding block.The angle that encompasses these new modes is defined by the followingformula:

$\alpha = \frac{\pi}{4} - \arctan\left( \frac{L_{\text{shorter}}}{L_{\text{longer}}} \right)$

wherein L_(shorter) and L_(longer) are the lengths of the shorter andlonger sides of the rectangular video coding block, respectively. Asillustrated in FIG. 9 , L_(shorter) = width and L_(longer) = height fora vertical orientation of the rectangular video coding block. The actualnumber of these modes may depend on the angle between neighbordirectional modes and the angle α defined by the above formula.

In the up-to-date version of the JEM software (version JEM-4.0), theaverage angle step between neighbor directional modes defined by anintra-prediction interpolation filter does not depend on the block sizeand equals:

$s = \frac{\pi}{64}$

Thus, in the case of uniformly spaced directional intra-predictionmodes, the number N of the newly introduced modes equals:

$N = \left\lfloor \frac{\alpha}{s} \right\rfloor = 16 - \left\lfloor {\frac{64}{\pi}\arctan\left( \frac{L_{\text{shorter}}}{L_{\text{longer}}} \right)} \right\rfloor$

wherein └˙┘is a floor operation.

In the embodiment shown in FIG. 9 , the number of reference samples isextended along the longer side, and it is not reduced for the shorterside. Therefore, the amount of intra-prediction modes that are availablealong the longer side (the angle that encompasses these modes is markedby a solid line) is increased, but the number of intra-prediction modesthat are available along the shorter side (the angle that encompassesthese modes is marked by a dashed line) is not decreased. Hence, thecardinality of the intra-prediction mode set is only increased while theaspect ratio

$R_{\text{asp}} = \frac{L_{\text{shorter}}}{L_{\text{longer}}}$

is decreasing. On the other hand, another approach to preserve theoriginal number of directional intra-prediction modes is also possibleaccording to another embodiment.

FIG. 10 shows a schematic diagram illustrating a preservation of acardinality of directional intra-prediction modes subject to an aspectratio of a given rectangular video coding block. As shown in FIG. 10 ,the amount of the directional intra-prediction modes added along thelonger side (the angle that encompasses these modes is marked by a solidline) may be equal to the amount of the directional intra-predictionmodes removed along the shorter side (the angle that encompasses thesemodes is marked by a dashed lines). Thus, the cardinality of theintra-prediction mode set remains the same as for square blocks.

According to an embodiment, whether to extend a set of availableintra-prediction modes or not can also depend on the availability ofreference samples because they are needed to generate anintra-predictor.

FIG. 11 illustrates an example of block partitioning and a correspondingtree structure by using quad-tree plus binary-tree (QTBT), wherein thenumber of available reference samples along a longer side is less thanits double length in a rectangular video coding block. As shown in FIG.11 , the quad-tree plus binary-tree (QTBT) partitioning frameworkproduces a partitioning, wherein the actual number of availablereference samples along a longer side is less than its double length asassumed in the above examples in FIGS. 9 and 10 . Therefore, theapproach for increasing the number of the directional intra-predictionmodes in the above examples may need to be adjusted according to anavailability of reference samples for the case of FIG. 11 .

FIG. 12 illustrates enabling or disabling a set of directionalintra-prediction modes subject to an availability of reference samplesof a given rectangular video coding block within the quad-tree plusbinary-tree (QTBT) partitioning framework, wherein a grey rectangle arearepresents a currently processed video coding block, square pixels withdiagonal stripes indicate available reference samples, and square pixelswith dots indicate unavailable reference samples. Disabling can e.g. beachieved by removing a respective directional intra-prediction mode fromthe set.

A fractional non-prediction area P of a rectangular video coding blockgenerated using interpolated reference samples may be calculated asfollows:

$\begin{matrix}{P_{\text{area}} = \frac{S_{\text{uncov}}}{S_{\text{block}}} = \frac{S_{\text{uncov}}}{L_{\text{shorter}} \cdot L_{\text{longer}}} = \frac{L_{\text{shorter}}^{2} \cdot \tan\gamma}{2 \cdot L_{\text{shorter}} \cdot L_{\text{longer}}} =} \\{= \frac{L_{\text{shorter}} \cdot \tan\gamma}{2 \cdot L_{\text{longer}}} = \frac{L_{\text{shorter}}}{L_{\text{longer}}} \cdot \frac{\tan\gamma}{2} = R_{\text{asp}} \cdot \frac{\tan\gamma}{2}}\end{matrix}$

wherein L_(longer) and L_(shorter) are the lengths of the longer andshorted sides of a rectangular video coding block, respectively, γ isthe angle of a given directional intra-prediction mode belonging to theextended set, S_(block) = L_(shorter) · L_(longer) is the area of arectangular video coding block to be predicted,

$S_{\text{uncov}} = \frac{L_{\text{shorter}}^{2} \cdot \tan\gamma}{2}$

is the non-prediction area, i.e. the area of the video coding block thatmay not be predicted using non-interpolated reference samples, as markedby stripes.

Therefore, the closer an intra-prediction direction is located to thediagonal marked by a dashed line, the larger part of an area thatremains may not be predicted using non-interpolated reference samples.In an example, the set of directional intra-prediction modes is notextended if the length L_(RSlonger) of non-interpolated referencesamples along the longer side is less than the double length of thelonger side:

L_(RSlonger) < 2L_(longer) .

If a set of directional intra-prediction modes is extended, it isdesirable to signal the newly extended modes, which may not beaccomplished using existing conventional mechanisms. For this purpose, a2-step signaling mechanism for the extension of directionalintra-prediction modes is set forth and explained in FIGS. 13 and 14 .

FIG. 13 illustrates a first step of a signaling mechanism for extensionof directional intra-prediction modes, wherein a set of extended modesis mapped to a conventional set of intra-prediction modes using amirroring procedure.

FIG. 14 illustrates a second step of a signaling mechanism for extensionof directional intra-prediction modes, wherein a one-bit flag is used todistinguish between conventional and extended directional modes. Theflag is assigned a value “0” for a conventional mode and “1” for anextended mode. Furthermore, the flag in the signaling mechanism is usedonly for those directional modes that are reflections of extended ones.

FIG. 15 illustrates a decoding process for a directional intra modeindex by applying a signaling mechanism. As shown in FIG. 15 , theextended modes of the directional intra-prediction are flagged with “1”,the conventional modes having a mapped mode are flagged with “0”, andthe other modes have no additional signaling value.

FIG. 16 shows a schematic diagram illustrating an implementation of asignaling mechanism applied in an encoding apparatus. In a firstprocessing step 1601 the index of the intra-prediction mode I_(IPM) isparsed from the bitstream. Thereafter, in processing step 1603 adecision is taken depending on whether the decoded intra-prediction modeis a directional intra prediction mode. In the case the signaling schemeis applied in the context of HEVC video coding, the intra-predictionmode is directional when I_(IPM) is greater than 1. If theintra-prediction mode is directional, in processing step 1605 a decisionis taken depending on whether the decoded intra-prediction mode isextended. The decoded intra-prediction mode is extended when I_(IPM) isgreater than Q[π/2+arctan(Width/Height)] and smaller than VDIAG_IDX,wherein Width and Height are the lengths of short and long sides of arectangular video coding block being decoded, and VDIAG_IDX is equal to66 according to embodiments of the invention. Then, the flag“ext_dir_mode_flag” is assigned to a value of 0 for the conventionalmodes which can have mapped extended code (see processing steps 1607,1609). A rate-distortion cost (RD-cost) is estimated for theconventional modes in processing step 1611. The flag “ext_dir_mode_flag”is assigned to a value of 1 for the extended modes (see processing steps1613, 1615). A rate-distortion cost (RD-cost) for the conventional modesis estimated in processing step 1617. The flag “ext_dir_mode_flag” isdetermined by finding the lowest rate-distortion cost (RD-cost) betweenthe conventional modes and extended modes in processing step 1619.

FIG. 17 shows a schematic diagram illustrating an implementation of asignaling mechanism applied in a decoding apparatus. In a firstprocessing step 1701 the index of the intra-prediction mode I_(IPM) isparsed from the bitstream. Thereafter, in processing step 1703 adecision is taken depending on whether the decoded intra prediction modeis a directional intra prediction mode. In the case the signaling schemeis applied in the context of HEVC video coding, the intra predictionmode is directional when I_(IPM) is greater than 1. If theintra-prediction mode is directional, in processing step 1705 a decisionis taken depending on whether the decoded intra-prediction mode isextended. The decoded intra-prediction mode is extended when I_(IPM) isgreater than Q[π/2+arctan(Width/Height)] and smaller than VDIAG_IDX,wherein Width and Height are the lengths of short and long sides of arectangular block being decoded, and VDIAG_IDX is equal to 66 accordingto embodiments of the invention. For extended directionalintra-prediction modes the value of the flag “ext_dir_mode_flag” isparsed from the bitstream in processing step 1707. According toembodiments of the invention this flag is introduced into the bitstreamto code whether to apply the disclosed mechanism to the prediction unit.In processing step 1709, a decision is taken to use either the extendedprediction scheme if ext_dir_mode_flag is equal to 1 (processing step1711 a) or the conventional prediction if ext_dir_mode_flag is not equalto 1 (processing step 1711 b), as provided by embodiments of theinvention, for obtaining the predicted signal. The decision inprocessing step 1709 is taken on the basis of the value of the flag“ext_dir_mode_flag”, which has been determined in processing step 1707.

The signaling mechanism is applicable to a wider spectrum of casesaccording to embodiments of the invention. For example, it can be usedto reduce a signaling overhead caused by an extended set of directionalintra-prediction modes used in Enhanced Intra-Prediction (EIP) techniqueproposed by Google for its VPX codec family. This EIP technique isneeded to improve the compression efficiency of intra-predicted blockswithin inter-predicted pictures. EIP is a two-pass mechanism forincreasing the number of available prediction directions, wherein blockswith good inter-prediction modes are initially encoded, and then intrablocks with access to more boundaries are filled in.

FIGS. 18(a) and 18(b) show schematic diagrams illustratingimplementations of a signaling mechanism applied to the EnhancedIntra-Prediction (EIP) mechanism. In the cases shown in FIGS. 18(a) and18(b), 4 (2π) and 3 (3π/2) sides of a video coding block are availablefor directional intra-prediction, respectively. Solid lines denotedirections from a main angle and dashed lines denote directions from acomplimentary angle. In both cases, the set of availableintra-prediction modes is more than for a conventional case.

As described above, the same 2-step signaling mechanism can be conductedto signal what angle the selected directional intra-prediction modebelongs to by using a one-bit flag. Firstly, a directional mode can bemapped onto the main angle if the directional mode is selected from thecomplementary angle. Secondly, the one-bit flag can be set to “ON” ifthe direction is selected from the complementary angle; otherwise, theflag can be set to “OFF”.

FIG. 19 shows a schematic diagram of an encoding apparatus 200 forencoding a rectangular video coding block comprising an intra-predictionapparatus 100. A decoding apparatus 300 can be implemented analogously.

While a particular feature or aspect of the disclosure may have beendisclosed with respect to only one of several implementations orembodiments, such a feature or aspect may be combined with one or morefurther features or aspects of the other implementations or embodimentsas may be desired or advantageous for any given or particularapplication. Furthermore, to the extent that the terms “include”,“have”, “with”, or other variants thereof are used in either thedetailed description or the claims, such terms are intended to beinclusive in a manner similar to the term “comprise”. Also, the terms“exemplary”, “for example” and “e.g.” are merely meant as an example,rather than the best or optimal. The terms “coupled” and “connected”,along with derivatives thereof may have been used. It should beunderstood that these terms may have been used to indicate that twoelements cooperate or interact with each other regardless whether theyare in direct physical or electrical contact, or they are not in directcontact with each other.

Although exemplary aspects have been illustrated and described herein,it will be appreciated that a variety of alternate and/or equivalentimplementations may be substituted for the exemplary aspects shown anddescribed without departing from the scope of the present disclosure.This application may cover any adaptations or variations of theexemplary aspects discussed herein.

Although elements in the following claims may be recited in a particularsequence with corresponding labeling, unless the claim recitationsotherwise imply a particular sequence for implementing some or all ofthose elements, those elements are not necessarily intended to belimited to being implemented in that particular sequence.

Many alternatives, modifications, and variations will be apparent tothose skilled in the art in light of the above teachings. Of course,those skilled in the art readily recognize that there are numerousapplications of the invention beyond those described herein. While thepresent disclosure has been described with reference to one or moreexemplary embodiments, those skilled in the art recognize that manychanges may be made thereto without departing from the scope of thepresent invention. It is therefore to be understood that within thescope of the appended claims and their equivalents, embodiments of theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. An intra-prediction method, comprising: obtaininga conventional directional intra-prediction mode based on signaledinformation; determining an extended directional intra-prediction modebased on the conventional directional intra-prediction mode for arectangular video coding block, wherein the rectangular video codingblock has a first side and a second side, and wherein a length of thefirst side is greater than a length of the second side; determining aplurality of available reference samples based on a direction of theextended directional intra-prediction mode; and intra-predicting pixelvalues of pixels of the rectangular video coding block based on theplurality of available reference samples.
 2. The intra-prediction methodof claim 1, determining the extended directional intra-prediction modefor the rectangular video coding block by mapping the conventionaldirectional intra-prediction mode.
 3. The intra-prediction method ofclaim 1, wherein determining the extended directional intra-predictionmode for the rectangular video coding block based on an aspect ratio ofthe rectangular video coding block and the conventional directionalintra-prediction mode.
 4. The intra-prediction method of claim 3,wherein the aspect ratio of the rectangular video coding block is$R_{\text{asp}} = \frac{L_{\text{shorter}}}{L_{\text{longer}}}\mspace{6mu},$where L _(shorter) and L_(longer) are the lengths of the second andfirst sides of the rectangular video coding block, respectively.
 5. Theintra-prediction method of claim 1, wherein the rectangular video codingblock is a coding unit, a prediction unit, or a transform unit.
 6. Theintra-prediction method of claim 1, wherein the conventional directionalintra-prediction mode is applicable to both square video coding blocksand non-square rectangular video coding blocks, and wherein the extendeddirectional intra-prediction mode is applicable to non-squarerectangular video coding blocks.
 7. The intra-prediction method of claim6, wherein the extended directional intra-prediction mode is notapplicable to square video coding blocks.
 8. An intra-predictionapparatus, comprising: a non-transitory computer-readable storage mediumstoring instructions; and one or more processors in communication withthe non-transitory computer-readable storage medium and upon executionof the instructions, causes the apparatus to: obtain a conventionaldirectional intra-prediction mode based on signaled information;determine an extended directional intra-prediction mode based on theconventional directional intra-prediction mode for a rectangular videocoding block, wherein the rectangular video coding block has a firstside and a second side, and wherein a length of the first side isgreater than a length of the second side; determine a plurality ofavailable reference samples based on a direction of the extendeddirectional intra-prediction mode; and intra-predicting pixel values ofpixels of the rectangular video coding block based on the plurality ofavailable reference samples.
 9. The intra-prediction apparatus of claim8, wherein the one or more processors in communication with thenon-transitory computer-readable storage medium and upon execution ofthe instructions, causes the apparatus to: determine the extendeddirectional intra-prediction mode for the rectangular video coding blockby mapping the conventional directional intra-prediction mode.
 10. Theintra-prediction apparatus of claim 8, wherein the one or moreprocessors in communication with the non-transitory computer-readablestorage medium and upon execution of the instructions, causes theapparatus to: determine the extended directional intra-prediction modefor the rectangular video coding block based on an aspect ratio of therectangular video coding block and the conventional directionalintra-prediction mode.
 11. The intra-prediction apparatus of claim 10,wherein the aspect ratio of the rectangular video coding block is$R_{\text{asp}} = \frac{L_{\text{shorter}}}{L_{\text{longer}}},$ where L_(shorter) and L_(longer) are the lengths of the second and first sidesof the rectangular video coding block, respectively.
 12. Theintra-prediction apparatus of claim 8, wherein the conventionaldirectional intra-prediction mode is applicable to both square videocoding blocks and non-square rectangular video coding blocks, andwherein the extended directional intra-prediction mode is applicable tonon-square rectangular video coding blocks.
 13. The intra-predictionapparatus of claim 12, wherien the extended directional intra-predictionmode is not applicable to square video coding blocks.
 14. Theintra-prediction apparatus of claim 8, wherein the rectangular videocoding block is a coding unit, a prediction unit, or a transform unit.15. The intra-prediction apparatus of claim 8, wherein the apparatus isa decoder.
 16. A non-transitory computer readable medium havingprocessor-executable instructions stored thereon for intra-prediction,wherein the processor-executable instructions, when executed,facilitate: obtaining a conventional directional intra-prediction modebased on signaled information; determining an extended directionalintra-prediction mode based on the conventional directionalintra-prediction mode for a rectangular video coding block, wherein therectangular video coding block has a first side and a second side, andwherein a length of the first side is greater than a length of thesecond side; determining a plurality of available reference samplesbased on a direction of the extended directional intra-prediction mode;and intra-predicting pixel values of pixels of the rectangular videocoding block based on the plurality of available reference samples. 17.The non-transitory computer readable medium of claim 16, wherein thedetermining the extended directional intra-prediction mode for therectangular video coding block comprises: determining the extendeddirectional intra-prediction mode for the rectangular video coding blockby mapping the conventional directional intra-prediction mode.
 18. Thenon-transitory computer readable medium of claim 16, wherein thedetermining the extended directional intra-prediction mode for therectangular video coding block comprises: determining the extendeddirectional intra-prediction mode for the rectangular video coding blockbased on an aspect ratio of the rectangular video coding block and theconventional directional intra-prediction mode.
 19. The non-transitorycomputer readable medium of claim 18, wherein the aspect ratio of therectangular video coding block is$R_{\text{asp}} = \frac{L_{\text{shorter}}}{L_{\text{longer}}}\mspace{6mu},$where L _(shorter) and L_(longer) are the lengths of the second andfirst sides of the rectangular video coding block, respectively.
 20. Thenon-transitory computer readable medium of claim 16, wherein theconventional directional intra-prediction mode is applicable to bothsquare video coding blocks and non-square rectangular video codingblocks, and wherein the extended directional intra-prediction mode isapplicable to non-square rectangular video coding blocks.